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H.E.S.S. discovers radio galaxy shining in gamma light

30.03.2009

An international team of astrophysicists, for the first time, has observed very-high-energy gamma radiation from a nearby active galactic nucleus, the radio galaxy Centaurus A.

The weak radiation has been discovered by the H.E.S.S. telescopes in Namibia, currently one of the most sensitive instruments of high-energy astrophysics. (The Astrophysical Journal Letters 695, L40-L44, 2009)

Active galactic nuclei are the most energetic objects in the Universe. Around the suspected supermassive black hole they harbour at their centre, charged particles (electrons and protons) may be accelerated to velocities close to the speed of light and ejected in oppositely directed jets. Centaurus A, located in the constellation Centaurus, is one of the brightest galaxies in the night sky and the nearest radiogalaxy with an active nucleus. Its proximity enables unique studies of the active centre and its surrounding. Centaurus A covers an area of the sky more than 100 times the size of the area of the full moon - but this extended structure only glows in radio frequencies with only the host galaxy being visible to the naked eye.

The telescopes of the High Energy Stereoscopic System (H.E.S.S.) in Namibia have now, for the first time, observed very-high-energy gamma-ray emission from Centaurus A. H.E.S.S. consists of four identical telescopes with 13 m mirror diameter and is operated by an international collaboration, coordinated by the Max-Planck-Institut für Kernphysik in Heidelberg. Ultrafast cameras record the weak blue flashes that arise when very-high-energy gamma-ray photons are absorbed in the atmosphere and create cascades of subatomic particles, so-called particle showers.

The high-energy gamma radiation from Centaurus A is so weak that more than 100 hours of observation time were needed to obtain a picture. The detected emission originates from the centre of the galaxy and the inner parts of the jets. With the current data, however, it is not yet possible to identify the exact origin of the emission. These gamma rays - a trillion times more energetic than visible light - are produced, it is thought, when particles, accelerated to extreme energies in the vicinity of a black hole, interact with radiation fields or the surrounding medium.

The detection of very-high-energy gamma rays from Centaurus A poses the more general question of whether such emission might be a common feature of active galactic nuclei. To answer this question, further observations of Centaurus A and of other active galactic nuclei are necessary. In that case, future instruments with higher sensitivity will be able to detect many more sources than previously anticipated and so better determine the processes involved.

A very large telescope with a mirror diameter of 30 m to extend the H.E.S.S. experiment is already under construction and will start observation in 2010. For the future, the European project Cherenkov Telescope Array (CTA) is planned. This gamma-ray observatory will consist of roughly 100 telescopes, leading to an improvement in sensitivity by a factor 10 compared to the current generation of instruments.

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